In recent years, along with a tendency to high brightness and whitening of a light emitting element such as a light emitting diode (hereinafter sometimes referred to as LED) device, a light emitting device employing LED for a backlight of a mobile phone, a large sized liquid crystal TV, etc. has been used.
However, along with a tendency to high brightness of a light emitting element, heat generated from a light emitting device is also increasing. Therefore, in order to be free from a decrease in the brightness of a light emitting element due to an increase in the temperature, a substrate for a light emitting element and an LED package (hereinafter sometimes referred to simply as a package) having high heat dissipation properties such that generated heat is quickly dissipated from a light emitting element have been desired.
As a substrate for a light emitting device, heretofore, an alumina substrate which has been used as an insulating substrate for a wiring board has been used in many cases, but an alumina substrate has a problem such as a low thermal conductivity of about from 15 to 20 W/m·K.
Further, as a substrate which replaces the alumina substrate, attention started being paid to an aluminum nitride substrate having a high thermal conductivity. However, an aluminum nitride substrate had a problem of a high cost of raw materials and a low coefficient of thermal expansion of from 4×10−6 to 5×10−6/° C., and thereby had a problem such that when mounted on a printed board having a coefficient of thermal expansion of at least 9×10−6/° C. as a universal product, connection reliability will be decreased by a large difference in the coefficient of thermal expansion. Further, since an alumina substrate and an aluminum nitride substrate are hardly sintered, high temperature firing will be required, thus leading to an increase in the process cost.
In order to solve such problems, a low temperature co-fired ceramic substrate (hereinafter sometimes referred to as an LTCC or an LTCC substrate) started being used as an LED package substrate.
An LTCC comprises glass and a ceramic filler such as an alumina filler, provides a large difference between the refractive index of the glass and the refractive index of the ceramic filler, provides a large area of an interface which reflects light since the number of the ceramic filler components dispersed in the LTCC is large, and further has a high reflectance since the thickness of the glass or the ceramic filler at both sides of the interface is larger than the wavelength of the light. Accordingly, the LTCC can efficiently reflect light from a light emitting device, and as a result, heat generation can be decreased.
Further, since the LTCC is formed by an inorganic oxide, it is free from deterioration by a light source unlike a resin substrate, and can stably maintain the color tone over a long period of time.
However, since the LTCC cannot be said to have a sufficiently high thermal conductivity, it is likely to cause a decrease in the brightness of a light emitting element. In order to solve this problem, heretofore, it has been known to reduce the thermal resistance by providing a through hole conductor for heat dissipation i.e. a thermal via which transfers heat from the LED to a heatsink on the opposite side of the substrate. Here, the through hole conductor is also called a via conductor.
FIG. 1 is a transcription of FIG. 2 in Patent Document 1 as an example of a conventional light emitting diode package. Just under an LED chip (LED element) 21 as a light emitting element, a through hole conductor (thermal via) 10 is located, and the LED chip 21 is attached on the thermal via 10 by means of an adhesive 29. In order to realize low thermal resistance from the LED to the rear side of a substrate, a via having a relatively large diameter is required. Since the amount of heat which can be transferred by a single thermal via is limited by its diameter, a large number of thermal vias are usually formed in the substrate.
The LTCC is usually prepared by adding a resin and the like to a raw material powder containing a glass powder and a ceramic filler (ceramic powder) as the main components and forming the mixture into a sheet, which is fired at a temperature of at most 1,000° C. to obtain a substrate. The above sheet-form product is called a green sheet, and processing such as cutting or punching is applied thereto as the case requires, a conductor is printed thereon by using a conductor paste, and usually a plurality of such sheets are laminated, followed by firing.
The LTCC can be prepared in such a manner that a powder of a silver conductor or a copper conductor having a high thermal conductivity is formed into a paste, which is e.g. embedded in a through hole provided on the green sheet, followed by co-firing with a silver conductor or a copper conductor, and accordingly a substrate provided with a through hole conductor excellent in heat dissipation properties can efficiently be produced.
Patent Document 1: JP-A-2006-41230